IN VITRO KNEE KINEMATICS FOLLOWING REMOVAL OF THE CENTRAL ONE-THIRD OF THE PATELLAR TENDON IN SHEEP (EORS AWARD FINALIST 2008)

Abstract

Infrapatellar Contracture Syndrome describes a postoperative complication characterised by a vertical migration of the patella due to Patella Tendon (PT) shortening and/or PT adhesion (PTA) formation. We investigated how removal of the central one-third of the PT influences both PT length (LP) and in vitro knee kinematics in 18 sheep divided into 3, 6, 12 and 24 week groups. At time of sacrifice the pelvis-lower extremities complexes were left in a supine position until rigor mortis set in. Limbs were CT-scanned (0.5mm) whilst frozen and LP measured (ProEngineer, PTC, MA). Specimens were fixed into a loading frame with 50N applied to the rectus femoris and knee kinematics obtained (Polhemus, VT). Bones and associated registration block portions of the receiver assemblies were CT-scanned (0.5mm), reconstructed, and imported into ProEngineer where coordinate systems were created in accordance with the Joint Coordinate System (JCS). Registration was accomplished by aligning models of the receiver assemblies with the reconstructed surfaces. Post-processing and statistical analysis (ANOVA) was performed using Matlab (MathWorks, MA) and data referenced to the contralateral controls.

No significant changes in LP were observed. The mean PT length ratio (LP/LC) in the 3 week group was 1.0028±0.004 (mean±SD). In the 6 week group this ratio had increased to 1.0282±0.0246, returning to 1.005±0.0035 at 12 weeks and back to 1.0159±0.0217 at 24 weeks. No PTA’s were observed. A significant proximal shift of the patella reflecting the increase in LP was observed which correlated well with a retardation of patellar flexion (r = 0.880, p< 0.001). A significant decrease in medial patellar tilt was also observed but was not coupled with changes in tibial rotation. Proximal and lateral tibial shifts were also detected.

The results of this study seem to suggest that the changes in knee kinematics and LP induced by removal of the central one-third of the PT do not recover 24 weeks post-operatively.